1. Field of the Invention
The present invention relates to a printing unit and a thermal printer.
2. Description of the Related Art
Hitherto, thermal printers are configured to perform printing by heating a printing surface of a recording sheet with heating elements of a thermal head to develop a color on the printing surface while feeding the recording sheet through rotation of a platen roller under a state in which the recording sheet is nipped between the platen roller and the thermal head. In those thermal printers, the platen roller is removable to facilitate work of replacing the recording sheet or the like.
This type of thermal printer (printing unit) includes a main body frame, a thermal head, a platen roller, platen bearings mounted on a roller shaft protruding from both ends of the platen roller in a coaxial state, bearing insertion grooves formed in the main body frame so that the platen bearings are inserted in a removable state, a lock lever (lock mechanism) movable from a locking position where the platen bearings are locked so as not to detach from the bearing insertion grooves to an unlocking position where the platen bearings are detachable from the bearing insertion grooves, and a lever biasing member (biasing member) configured to bias the lock lever constantly toward the locking position. The platen bearings are pressed against inner peripheral end surfaces of the bearing insertion grooves by the lock lever located at the locking position, to thereby determine the positions of the platen bearings with respect to the main body frame. In this manner, the removable platen roller is positioned with respect to the main body frame.
In the above-mentioned printing unit, however, an external force caused by drop impact or the like may be applied to the main body frame or the lock arm to distort the respective components, resulting in decrease in holding force of the lock arm for the platen roller. Thus, there is a risk of such trouble that the platen roller may drop off from the main body frame. This type of printing unit is generally mounted in a housing having an openable and closable cover, but the strength of the housing is not set extremely high in consideration of weight and cost. Thus, when the thus housing drop may occur, an inertial force and a force caused by distortion or twist are generated in the housing due to the drop impact, and those forces are applied to the main body frame or the lock arm so that the above-mentioned trouble may occur. Further, when the platen roller drops off in this printer housing, the cover is opened so that the device may be damaged or the roll sheet may drop off, resulting in inconvenience to the user of the printer.
In this type of printing unit, a fit tolerance is set between a pivot shaft of the lock mechanism and a shaft hole of the main body frame through which the pivot shaft is inserted. Therefore, when an external force is applied to the printing unit, there is a risk in that the pivot shaft of the lock mechanism may be moved within a range of the fit tolerance, namely the pivot shaft of the lock mechanism may be rattled. When the pivot shaft of the lock mechanism is moved from a desired position, the holding force to be applied from the lock mechanism to the platen roller and the direction of application of the holding force may become unstable, thereby causing a risk of misalignment of the platen roller from the thermal head or the like. As a result, printing failure such as printing blurs and periodic unevenness, namely printing unevenness to be caused by a change in contact state between the heating elements and the recording sheet along with periodic vibrations may occur.
In view of the above-mentioned matters, a printing unit and a thermal printer capable of stabilizing the position of the platen roller with respect to the thermal head to secure print quality are demanded in this technical field.